1. Field of the Invention
The invention is directed to a single mode optical waveguide fiber for use in telecommunication systems and more particularly, a waveguide fiber which reduces non-linear dispersion effects, and combines low attenuation with a large effective area.
2. Technical Background
Wavelength division multiplexing techniques are typically required in telecommunication systems used in metropolitan area networks. Data rates in such systems are high so that the bandwidth demand is great to provide high capacity transmission capabilities. The single mode optical waveguide fiber that has been the primary fiber employed for use within metropolitan area networks has significant drawbacks since it cannot be used with direct modulation DFB (distributed feedback) lasers which are less expensive and easier to manufacture. Using a directly modulation DFB laser causes degradation in signal quality due to significant linear dispersion as well as non-linear dispersion effects. Thus, the more costly externally modulated transmitters or electro-absorption DFB lasers must be used.
To provide a suitable waveguide fiber for a multiplex transmission, the total dispersion should be low, but not zero, and have a low slope over the window of operating wavelengths. Maintaining the total dispersion non-zero over the operating wavelength band limits the non-linear dispersion effect of four wave mixing.
Several other non-linear optical effects must be considered when designing a waveguide fiber in order to reduce dispersion and yet be compatible for use with direct modulation DFB lasers utilizing multiplexing techniques. These non-linear optical effects include four-wave mixing, self phase modulation, cross phase modulation, and non-linear scattering.
Generally, an optical waveguide fiber having a large effective area, Aeff, reduces non-linear optical effects, including self phase modulation, four wave mixing, cross phase modulation, and non-linear scattering processes, all of which can cause degradation of signals in systems utilizing multiplexing techniques. A waveguide fiber having a segmented core can generally provide a large effective area while limiting the non-linear optical effects.
The mathematical description of these non-linear effects includes the ratio, P/Aeff, where P is the optical power. For example, a non-linear optical effect can be described by an equation containing a term, exp [Pxc3x97Leff/Aeff], where Leff is effective length. Thus, an increase in Aeff produces a decrease in the non-linear contribution to the degradation of a light signal.
The requirement in the telecommunication industry for greater information capacity and reduced cost of light sources, such as the directly modulated DFB lasers, has led to a reevaluation of single mode optical waveguide fiber index profile design. The focus of this reevaluation has been to provide optical waveguides which:
reduce non-linear effects such as those noted above;
have lower dispersion than standard single mode fibers;
may be used with all band wavelength division multiplexing applications with low-water peak attenuation;
are compatible with 1310 nm systems; and
retain the desirable properties of waveguides such as high strength, fatigue resistance, and bending resistance.
Waveguide designs which also are relatively easy to manufacture and which permit management of dispersion are favored, because of their low cost and added flexibility. The designs described herein are well suited to a dispersion managing strategy in which the waveguide dispersion is varied along the waveguide fiber length to cause the total dispersion to alternate between positive and negative values.
U.S. Pat. No. 4,715,679, to Bhagavatula, incorporated herein by reference as if fully set forth herein in its entirety, discloses and describes the concept of adding structure to the waveguide fiber core by means of core segments, having distinct profiles to provide flexibility in waveguide fiber design. The segmented core concept can be used to achieve unusual combinations of waveguide fiber properties, such as those described herein.
This invention meets the need for a singe mode optical waveguide fiber that offers the benefits of a relatively lower total dispersion (the sum of waveguide and material dispersion) with a relatively large effective area.
The invention relates to a single mode optical waveguide fiber including a segmented core. Each of the segments is described by a refractive index profile, a relative refractive index percent, and an inner and an outer radius. The optical waveguide fiber further includes a clad layer surrounding and in contact with the core, and having a refractive index profile.
In a preferred embodiment, the index profiles are selected to provide total dispersion at a wavelength of about 1550 nm of within the range of about 11 ps/nm-km to about 14 ps/nm-km, total dispersion slope at a wavelength of about 1550 nm of within the range of about 0.045 ps/nm2-km to about 0.055 ps/nm2-km, an effective area greater than or equal to 60 xcexcm2, and attenuation at a wavelength of about 1550 nm of less than or equal to 0.22 dB/km.